Phase contrast (PC) is an exciting emerging x-ray imaging technique which removes most of the limitations of conventional x-ray imaging. Phase contrast can be applied to all fields of x-ray imaging, i.e. medical (diagnosis and treatment planning/delivering/monitoring), industrial (inspections, non-destructive testing) as well as to homeland security (security inspections). Basically all these fields would strongly benefit from the introduction of a reliable PC technique, as this would result in a strongly increased visibility of all details and in the possibility of detecting features which are invisible to conventional techniques.
A review of PC imaging is provided in R. Lewis, Medical phase contrast x-ray imaging: current status and future prospects, Phys. Med. Biol. volume 49 (2004) pages 3573-83.
Unlike more conventional techniques, which are based on absorption, PC is based on phase shift effects. The term responsible for phase effects is much bigger (˜1000 times) than the term accounting for absorption, hence the dramatically increased sensitivity of PC.
There are three conventional ways of exploiting phase effects. One is to optimize the sample-to-detector distance and detect the interference pattern which results from the phase perturbations: this approach is called free-space propagation or in-line holography. The results provided by this approach are strongly dependent on the source characteristics, which make the results obtainable with conventional sources rather poor. Excellent images are obtained with synchrotron radiation, but in order to transfer the technique to conventional sources severe tradeoffs on image quality and/or exposure times have to be accepted.
Examples of this approach may be found in A. Snigirev et al, On the possibilities of x-ray phase contrast microimaging by coherent high-energy synchrotron radiation, Rev. Sci, Instrum. volume 66 (1995) pages 5486-92, and S. W. Wilkins et al Phase-contrast imaging using polychromatic hard x-rays, Nature volume 384 (1996) pages 335-8.
A second approach involves the use of interferometers. Traditionally these are obtained by proper cutting of perfect crystals, which leads to a number of problems—only very small fields of view can be observed, the required beam has to be strictly parallel and monochromatic, and the radiation dose is delivered ineffectively. This makes the approach very difficult to apply in most situations. An example of this approach is that described in A. Momose et al Phase-contrast x-ray computed tomography for observing biological soft tissues, Nature Medicine volume 2 (1996) pages 473-5.
Recently, an approach based on grating interferometers was devised, which solves some of the problems related to the use of conventional, crystal-based interferometers. This approach is described in F. Pfeiffer et al Phase retrieval and differential phase-contrast imaging with low-brilliance x-ray sources, Nature Physics 2 (2006) 258-61.
However, this approach has limitations also: the interferometers are obtained by sophisticated microfabrication techniques currently allowing a maximum field of view of 5-6 cm, dose is delivered ineffectively, the technique is sensitive to phase effects in one direction only, it is necessary to step the gratings in at least four different positions to acquire a single image, and the spectral bandwidth of the radiation beam must be smaller than 10%.
The third approach is based on the fact that the distortions of the x-ray wavefront due to phase shift result in local microvariations in the x-ray direction. In other words, after exiting the imaged sample, the direction of the x-rays has changed by a few tens of microradians, which is an effect that can be detected and translated into image contrast.
This is done using an analyzer crystal which, being characterized by a very narrow reflectivity curve, allows the translation of angular deviations into intensity differences. Examples of this approach are provided in V. N. Ingal and E. A. Beliaevskaya X-ray plane-wave topography observation of the phase contrast from a non-crystalline object, J. Phys. D: Appl. Phys. volume 28 (1995) pages 2314-7, and D. Chapman et al Diffraction enhanced x-ray imaging, Phys. Med. Biol. volume 42 (1997) pages 2015-25.
This allows a very flexible approach (the system sensitivity can be changed by changing the crystal orientation) resulting in extremely high image quality, in most cases higher than that provided by all other approaches mentioned here.
However, the necessity of relying on a perfect crystal strongly limits the third approach's applicability, for four main reasons:                1) The system requires monochromatic, parallel radiation. This makes it the perfect tool for imaging with synchrotron radiation, but makes it extremely ineffective when a commercial x-ray source is employed. The result is an increase in the exposure time of possibly two or more orders of magnitude.        2) The dose is delivered inefficiently. The crystal absorbs a considerable fraction of the x-rays after they have transversed the sample. Increased doses are of course a particular problem in medical applications.        3) The system is highly sensitive to environmental vibrations: a change of 1 microradian in the crystal orientation is enough to affect image quality        4) The system is intrinsically sensitive to phase effects in one direction only.        
Thus, all three approaches have their disadvantages.
Another experimental approach uses synchrotron radiation, as described in A. Olivo et al An Innovative Digital Imaging Set-up Allowing a Low-Dose Approach to Phase Contrast Applications in the Medical Field, Med. Phys. volume 28 (2001) pages 1610-1619.
In these experiments it was observed that by illuminating with x-rays only the edge of the active surface of a line of pixels, it is possible to achieve a high sensitivity with respect to very small angular deviations in the photon direction.
Unfortunately, this experimental approach is difficult to convert to a commercial system for a number of reasons. Firstly, the approach inherently needs a flat x-ray sheet such as available from a synchrotron, and this is not available from conventional sources. The use of a synchrotron delivers highly collimated x-rays, and approaches using such radiation are difficult to convert to conventional sources for which beam divergence is a real issue. The use of a slit would greatly reduce the output x-ray intensity, making long exposure times necessary. Further, the approach does not work with conventional two-dimensional image detectors with an array of pixels which makes the approach incompatible with most existing equipment. Moreover, scanning across a sample to build up an image from a single line of detectors makes the process even slower and also makes it very difficult to maintain alignment. Such scanning is thus not compatible with commercial equipment where dosage limits and the timescale to record data are significant factors for example for use in medical or security applications.
The scientific community involved in x-ray imaging research fully agrees on the fact that phase contrast imaging can create a big change in x-ray imaging. To the best of the inventors' knowledge, up to now only two systems based on phase contrast imaging have been commercialized and they both suffer from limited applicability and/or limited improvements in image quality, for reasons discussed above.
Accordingly, there remains a need for an improved method of phase contrast imaging.